Messages

106

The RF Workbench / Re: Beginner class D design

« on: November 30, 2018, 0244 UTC »

Iron saturates at low frequencies, HPF your audio @100Hz.

I'd understand PWM if I were you, it's so simple you'll wonder why you ever used any other method.

My modulator probably weighs about 2oz.

The filter would weigh a bit but only 2-3lb.

You're building Class D RF so why not Class D audio too?

Str 

I can understand that the iron core might be saturating, but this doesn't seem like the case. When an 8 ohm load is on the transformer, it's pretty flat down to 100 Hz, but NOT when it's connected to the transmitter.

Let me know when you have a PWM board ready and I'll order it. It's nice to make use of old things around the shop though and that's why I'm testing the transformer first as I have several of them laying around. BTW, my audio amp is a class D (still less efficient than PWM though).

How are you measuring the impedance?  Stretchy is right, as frequency decreases, inductive reactance will go down, more so if the transformer is not tape wound with hypersil type steel.

+-RH

I am using an LCR meter that has resistance measurements at 100Hz, 120Hz, 1KHz and 10KHz. And I don't doubt the measurements because the amp shuts itself down unless the bass is equalized in the audio. When hooked up to an 8 ohm load, this is no longer the case. So the problem is only with the TX connected.

The HPF idea seems like not a bad one. I already tried that with an equalizer, but maybe I need more aggressive filtering like with a real HPF. That would probably work, but it seems like I'm dodging the real problem.

107

The RF Workbench / Re: Beginner class D design

« on: November 28, 2018, 1036 UTC »

You may want to take a look at this paper as it covers some of the problems of using transformer modulation at the few hundred watt level.
http://amfone.net/Amforum/index.php?action=dlattach;topic=30798.0;attach=31360

+-RH

I've already read that article. It gave me most of the info I have now about the transformer, but I'm still not sure why the impedance is so low at low frequencies and how that can be rectified 

I've discovered that the problem is not the actual transformer. If I connect an 8 ohm load to the transformer, I can see a pretty flat load on the other winding (not perfect, but no where near as bad as when it's connected to the TX). What determines the resistance here and why is it not flat? I tried changing the cap next to the RF choke, but it made no difference.

I know this is probably hard to determine without knowing the actual circuit. Also, this is a preliminary question and I'm not even sure it will be a problem when I get the amp I'm building, but it could be because the designs are very similar.

108

The RF Workbench / Re: Beginner class D design

« on: November 28, 2018, 0139 UTC »

It doesn't make sense to try to modify something that has an antiquated and overly complicated design. Using the modern technologies of DDS and class E or D doesn't seem to be a part of commercial ham radios. Maybe I'm wrong because I don't own any ham gear, but that's what it looks like to me.

That aside, the PCBs for my project have shipped and the FETs are already here. I will keep this thread updated with the progress when I begin the build.

I also have two new questions:

1. I tested a modulation transformer on a small class D (not the one I'm building). I've encountered a problem with the transformer. It is a 1:1 115V toroidal transformer. I don't know the VA, but it weighs about 15 kgs. I've read of using toroidal transformers as modulation transformers before and it looks like there are hams doing this and that it works well. Now we get to the problem: How do they not overload the audio amplifier when the ohms drops so low at the low end of the audio spectrum? If I look at the resistance at 100 Hz, it is 1 ohm and at 1 KHz, it is 6 ohm. So how can I get any bass into my signal without overloading the audio amplifier. Do I just need to filter the bass a certain amount?

2. Can I stack ferrite cores on the output transformer? For example, if I have 8 FETs, can I have four cores on the bottom and four on the top and then run the wire through all of them similar to if they were lined up? In the designs I've looked at, it seems like there is always one core per FET. Is this for power handling reasons or is it because the matching needs to change for each additional FET? (ie could I get away with four cores on an 8 FET amp, given that the power level is not too high or is it for matching?)

109

The RF Workbench / Re: Beginner class D design

« on: November 17, 2018, 1751 UTC »

Hearing what you know, I'd be interested to see some pictures of your design. Do you have any?

If not, then maybe you could give me a general idea of the layout. Are you using PCBs? How do you make connections (other than short as possible)? etc. Or maybe just some tips or things that you did wrong during the process.

I'd really appreciate it, but I understand if you want to keep it a secret.

110

The RF Workbench / Re: Beginner class D design

« on: November 17, 2018, 1726 UTC »

1 driver per fet will be required above 3 mhz in class D.  Low RDSon is required for class D as well, I would stick to the lower R fets.  I also don't like the thin and windy traced on the drains.  This should be a large plane, not a trace.  You will have a lot of ringing problems due to the stray inductance.  Areas like this I also make sure there is no copper on the back side to create stray capacitance which will load the output unnecessarily.  Same thing with the gate traces, large and fat equals lower inductance and better waveforms.

Little things like that make a BIG difference!

+-RH

OK, that makes sense. I don't think I can fix the drain traces though. There's just not enough space on there. I think I'll attach them to a raised piece of copper clad board. I've seen this somewhere before. It shouldn't be a deal killer though, right? I mean at these frequencies, I wouldn't expect a trace like that to have any level of inductance that would cause problems. Most inductors at these frequencies use toroids, so a little loop shouldn't do much. That's just speculation though. I'm new to this and you do know more, so...

In terms of the 1 FET per driver. I did already know that, but then I saw this guy demoing his class D transmitter. It has 4 FETs and two drivers and makes 200W carrier on 40m. His 2 FET version made 100W carrier, so it doesn't seem like the drivers are limiting it. I'm also using the TC4452 and he is the lower amp TC4422.
https://www.youtube.com/watch?v=QnM2Uum-_Cs

111

The RF Workbench / Re: Beginner class D design

« on: November 17, 2018, 1259 UTC »

So I've made some progress. I decided to just make a pro PCB instead of trying to make a dead bug. The PCBs from china are cheap enough anyway.

So here is what I made. It is based on the schematic I posted earlier, but I added spaces for 4 extra FETs. Let me know what you think or if you see any errors.

I created the board in Sprint Layout which I would highly recommend. CAD software is overly complicated for these types of designs imo (and I don't know how to use it .

I'm curious to see how well this works compared to class E. I know it won't be quite as good, but I'm more interested in bandwidth than obtaining max power from each device.

https://drive.google.com/open?id=1p_pzBloYqnlwnSdilyqJNou5V1moF8OQ

I decided to get two sets of mosfets: C2M0160120D and C2M0280120D

One has less capacitance, but lower current handling. As I understand it, the lower the capacitance, the higher the potential switching speed. I'm unsure of what is better though, more amps, or less capacitance. I find that the higher amp devices have more capacitance. So what is the higher priority? Amps could make up for the greater cap, but the less caps will make it faster. Not sure what's better.

112

The RF Workbench / Re: Beginner class D design

« on: November 14, 2018, 1737 UTC »

Thanks for your responses.

OK, so I've made some progress reading some more Class D/E schematics I could find out there. I came to find that the FAT5 from shortwaveradio.co.uk has a huge PDF with a ton of info. None of the other sites/documents I could find were nearly as thorough.

So I've compiled some info and made a schematic of my own from the bits I could find.
It is linked at the bottom. Please let me know if you see any errors. Some things I'm not clear on:

The capacitors on the drains - are they correct?
Not sure why they used 8V regulators, wouldn't 12V be better and more standard? The datasheet on these drivers also shows a slight improvement in switching speed with more volts.
I want to keep it simple to test it, so I want to try and use the old fashioned mod transformer and audio amp. I should probably know this, but: What turns ratio would I need?

https://drive.google.com/open?id=1mbt60zDB50rt65chP51dezb1KN28foh5

113

The RF Workbench / Beginner class D design

« on: November 12, 2018, 0104 UTC »

As I am building the transmitter discussed in this thread, I decided to document the progress on my blog. If you're looking to build your own class D, check it out. There is a massive archive of PDFs and schematics in there as well as an in depth explanation. I am not as experienced as some here, but I always update any inaccuracies. I also approach the topic from a novice's perspective.

http://dosaidsoft.com/wp/category/radio/class-d/

Original message:

I am trying to gain a better understanding of a class D transmitter, and while I do understand most of the concepts, I need a little clarification.

So I am probably going to start with this design: http://www.w1vd.com/40M375WclassDRev2.0.pdf
Yes, I know it's old. And I think there might be some parts left out. It looks like there is an unlabeled resistor at the gate of each FET.

First thing I am not clear on is if the exciter needs to be biphasic in order to make a class D or E amp. You can see in the schematic I linked that it has phase 1 and 2 input and it says 12V, but how many amps does it need (on the input)? Can I build half of the amp and use a normal single output DDS module? I am not sure how to make a two phase exciter. I think class D is more broadband, so I want to go with that. I'm still not even exactly sure what the difference is between D and E despite them being described so often. I never find a clear difference.

So when I have the amp and exciter built, I think I'll start trying to drive it with a mod transformer instead of PWM just to start because it's easier. So how many ohms do I need to match right at the drain of the FETs? Is it 50? No, probably not because that's after the matching right? So if I use a generic audio amp, what transformer ratio do I need? I think I'll use a microwave oven transformer. I have one that has all coils removed, so it's just a bare core I can wrap around.

If I need a two phase exciter. Where can I find one? I do know about the classeradio.com guy, but he only sells the PWM and not the exciter. He didn't respond anyway. Probably because I'm not ham.

And, yes, Stretchyman, I know you're here too. Your transmitter is tempting, but I don't want to just buy it without understanding how it works because that gets annoying in the long run.

Would you mind explaining a little to me? Mostly on how your DDS module works (I assume you use DDS). Is it a two phase thing or is it essentially just a 9850 and a PIC on a board - like what you can get on eBay? I would really like to see the schematics and also if you can clarify how the exciter module works. If you'd like a small payment for your schematics, I'd be willing. I'd just buy the whole thing if you can tell me how to change it for more bandwidth.

If I was building it myself I'd have used Class D for more bandwidth, so it's not perfect from my standpoint. What about your amp makes it Class E and could it be changed to D? I've seen a class D 100W amp that was using two SiC devices and it works 3-8 MHz. What makes yours only 200 Khz range?

I know that's a lot of questions and I am being very critical of the design. I know it is better than most others out there given that it's using the newest GaN and drivers. I hope you understand I'm am trying to learn w/o just buying stuff and wiring it up. That's too easy for this hobby and it makes it boring.

114

The RF Workbench / Re: PWM a BLF188XR?

« on: November 10, 2018, 1138 UTC »

BTW: I have a 20W PEP amp with 2x RD15HVF1 and the BLF188 (which I haven't finished constructing yet). I plan to drive the BLF with the former at reduced power.

I am using the 9850 DDS board I got from Germany on eBay (forgot the name). It has AM modulation built in. I think it is rf2017.

I will test it out soon, but I need a water cooling block and I'm trying to decide if I should buy or build one. I think I'll build it because I spent a lot of time looking for a good one. Not just size wise, but also with known thickness and tube pattern, so I know I am not going to puncture it when I drill the holes to mount the BLF. I'd rather not used forced air. It will just be bulky and loud. I already have a good radiator which should be more than capable.

115

The RF Workbench / Re: PWM a BLF188XR?

« on: November 10, 2018, 1130 UTC »

No it won't work as its a LINEAR AMP.

As soon as you reduce the voltage it will cease to function!

You need Class E which is way more efficient, simpler, cheaper etc etc.

They use those devices you quoted as they are modern, i.e. available, are cheap and are very easy to drive and have a low on resistance. There's no need to use GaN as they're for much higher (RF) frequencies. No real need to use SiC either unless the voltages are very high. Any crappy ol' FET will work as a PWM output as the frequency is only 200KHz or so....you just DONT want Ohms of on resistance as with highish current flow you'll drop volts and generate a lot of heat. SiC FETs can be use with high voltages keeping the current low, I2R is what generates loss.

With a linear amp you're best directly modulating the DDS and amplifying that to 3W to drive the linear. If you check my old posts I mention directly modding a AD9850 and there's a good article on the web about such..

Bought the same amp as you to do said build! You may beat me to it! Was also going to make a 3W SSB generator to drive it too!

Keep us posted!

Str.

Thank you for the response, and if you don't mind, see my other post in your thread about your 400W PWM.

What I meant to say about the BLF188 is: Why don't they use it in Class E configuration? Is it not possible? Is it the on resistance you where talking about?

116

The RF Workbench / PWM a BLF188XR?

« on: November 10, 2018, 0744 UTC »

This may sound like a stupid question to those who know more about RF engineering, but...
Why have I never seen an RF MOSFET PWM modulated or other high level modulated?
They are always used as linear amplifiers.

I have a BLF188XR amplifier board and I was wondering if it is possible to modulate it this way? Is low level modulation the only option with this?
Why does every PWM use GaN or SiC or those IRFP (whatever they're called)?

117

For Sale / Wanted / Barter / Re: 400W RF GENERATOR

« on: November 10, 2018, 0531 UTC »

I'm am interested in this transmitter, but I'm more interested to see the schematics than to buy it. I've learned I will always want more power and upgrades, and for that, it's better for me to understand the design first. Would you be willing to offer the schematics?

I'm also curious about the bandwidth of this design. Could a DDS module from ebay with 9850 and free tuning 2-30 MHz be used instead of channels? Of course, it probably can't go all the way to 30 MHz. You say 200 KHz segment, so it's narrowband (ie not balun match)?

118

North American MW Pirate Radio / UNID 1720 AM ~0445 UTC 11 Oct 2018

« on: October 11, 2018, 0532 UTC »

Did anyone hear it?
Begins with a carrier on and off and then music.

119

General Radio Discussion / Re: expanded FM band in Americas

« on: August 30, 2018, 0442 UTC »

I remember reading somewhere that the FCC thought about extending the Fm band after analog tv was shut off but there wasn’t enough public interest.

I've heard people say multiple times that since analog TV is gone, then those frequencies should be free. Maybe I'm misunderstanding, but I always thought that the digital stations used the same frequencies for broadcasting that the old analog stations used. Am I correct? If not, then what frequencies are they using?

120

Equipment / Re: Cuthbert AM Stereo C-Quam Transmitter Kit! Finally!!!!!

« on: July 22, 2018, 0844 UTC »

Yeah, no kidding! Just how hard would it be to write up software for SDR's to be able to have C-QUAM capabilities? I am the last person to know this since I almost failed out of computer electronics in college, since I hated machine language.

For near the same price as this you could get a HackRF. With that, you can pretty much modulate any type of signal you want. Sure you could do c-quam with it, but I don't know how. It doesn't require assembly language programming. It uses the gnuradio software which is like a flow chart. Not sure how well it works on AM band though. Some say it works down to 10 MHz and others say 1 MHz.